It occurs naturally in citrus fruits. In biochemistry, it is an intermediate in the citric acid cycle, which occurs in the metabolism of all aerobic organisms.
It's used in foods and cosmetics, and sometimes as medicine. Malic acid is sour and acidic. This helps to clear away dead skin cells when applied to the skin. Its sourness also helps to make more saliva in people with dry mouth.
Because of its acidic, sour-tasting nature, citric acid is predominantly used as a flavoring and preserving agent, especially in soft drinks and candies. It's also used to stabilize or preserve medicines and as a disinfectant. Citric acid is a compound originally derived from lemon juice.
